1. Field of the Invention
The present invention relates to a cathode ray tube, and particularly to a cathode ray tube capable of optimizing a grade of color purity of a screen by improving strength of a shadow mask and thus preventing deformation caused by an external impact or the like.
2. Description of the Conventional Art
A cathode ray tube is a device for converting an electric signal into an electron beam and emitting the electron beam to a phosphor screen to realize an image. The cathode ray tube is widely used in the conventional art since excellent display quality is achieved at an affordable price.
A cathode ray tube will be explained with reference to attached drawings. FIG. 1 is a schematic view showing an example of a cathode ray tube of the conventional art. As shown in FIG. 1, the cathode ray tube includes a panel 3 of a front glass; a funnel 2 of a rear glass engaged to the panel 3 for forming a vacuum space; a phosphor screen 13 deposited on an inner surface of the panel 3 and serving as a phosphor; an electron gun 6 for emitting an electron beam 5 which makes the phosphor screen 13 emit light; a deflection yoke 7 mounted at an outer circumference surface of the funnel 2 with a predetermined interval for deflecting the electron beam 5 to the phosphor screen 13; a shadow mask 8 installed at a constant interval from the phosphor screen 13; and a mask frame 9 for fixing and supporting the shadow mask 8. The cathode ray tube also includes an inner shield 10 extending from the panel 3 to the funnel 2 for shielding external terrestrial magnetism and thus preventing deterioration of color purity by the magnetism; and a reinforcing band 12 arranged at an outer circumference of the panel 3 for distributing stress generated from the panel 3 and the funnel 2.
As shown in FIG. 2, the shadow mask 8 includes a perforated portion 8b having a certain curvature corresponding to curvature of the inner surface of the panel 3 and having a plurality of electron beam passing apertures 8a through which the electron beam 5 passes; and a skirt portion 8c extended from an outer circumference of the perforated portion 8b in a tube axis direction for being fixed at the mask frame 9.
As shown in FIG. 3, the apertures 8a of the shadow mask 8 is circular in shape so that a horizontal dimension (Sh) and a vertical dimension (Sv) are identical.
Also, as shown in FIG. 4, in the electron beam passing apertures 8a, a width of an electron beam outgoing portion 81a (panel side) of the electron beam passing aperture 8a is tapered so as to be larger than that of an electron beam incoming portion 82a (electron gun side) in order to prevent a diffusion of an electron beam 5 passing therethrough. In order to correspond to an incidence angle of the electron beam, the tapered size becomes gradually large from the central portion toward the peripheral portion of the shadow mask 8.
In the conventional cathode ray tube, the electron beam 5 emitted from the electron gun 6 is deflected by the deflection yoke 7, passes through the plurality of apertures 8a of the shadow mask 8, and lands on the phosphor screen 13 formed at the inner surface of the panel 3. Accordingly, the deflected electron beam 5 makes the phosphor formed at the phosphor screen 13 emit light, thereby achieving an image.
Performance of a cathode ray tube can be determined by various factors. In this regard, color purity of an implemented image is one of the most important factors of the cathode ray tube, and the color purity is greatly affected by deformation of the shadow mask 8 caused by an external impact, in most cases.
Specially, since the tapered size of the aperture 8a of the shadow mask 8 becomes large from the central portion toward the peripheral portion of the shadow mask 8, a volume and a weight of the shadow mask 8 gradually decrease from the central portion of the shadow mask 8 toward the peripheral portion thereof, and thus the strength of the peripheral portion of the shadow mask 8 is lower than that of the central portion of the shadow mask 8.
Accordingly, in case that an external impact such as dropping occurs on the cathode ray tube, and especially in case that the panel 3 drops toward the ground, the shadow mask 8 is vibrated in the tube axis direction based on the surface thereof, and relatively great vibration is caused at the central portion of the shadow mask 8 than at the peripheral portion due to the relatively great mass of its central portion. Thusly, deformation occurs at the peripheral portion of the shadow mask 8, which has relatively low strength.
Also, in case that the whole size of a cathode ray tube is relatively large, a shadow mask 8 disposed therein becomes more sensitive to an external impact, Thusly, a shadow mask 8 for a large scale cathode ray tube can be permanently deformed by sudden deformation even under a small impact, so its performance is deteriorated.